1. Field of the Invention
The present invention relates to a liquid crystal display (LCD). More particularly, the present invention relates to an LCD having a modified electrode array.
2. Description of the Related Art
Generally, an LCD is a display having two substrates and a liquid crystal layer therebetween. A plurality of electrodes are formed on the inner surfaces of either or both the substrates, a pair of polarizers are attached to the outer surfaces of the substrates, and the liquid crystal layer serves as an optical switching medium. When a potential difference is applied to the electrodes, liquid crystal molecules are re-arranged due to the potential difference, and the re-arranged liquid crystal molecules scatter the incident light, which have passed through one of the polarizers, or change the transmission characteristics of the light, thereby controlling the transmittance of the light out of the other polarizer (which is usually called an analyzer) and displaying images.
As an example of a conventional LCD, U.S. Pat. No. 5,576,861 discloses a twisted nematic LCD (TN-LCD) where an upper electrode and a lower electrode formed respectively on the inner surfaces of upper and lower substrates and a nematic liquid crystal material is injected therebetween, and where the liquid crystal molecules are twisted with being parallel to the substrates. In the above LCD, the potential difference between the two electrodes generated by applying voltages to the upper and the lower electrodes yields an electric field perpendicular to the substrates. The liquid crystal molecules are re-arranged such that the torque due to the dielectric anisotropy and the torque due to the aligning treatment is balanced with each other. The torque due to the dielectric anisotropy forces the long axes of the liquid crystal molecules to be parallel to the field direction, and the magnitude of this torque depends on the intensity of the electric field. The elastic torque generated by the aligning treatment such as rubbing forces the long axes of the liquid crystal molecules to be parallel to a predetermined direction. When the director of the liquid crystal twists by 90 degrees on going from the lower electrode to the upper electrode, and the polarization directions of the polarizers are perpendicular to each other, the polarization of the incident light, in absence of the electric field, rotates by 90 degrees, and thus the light passes through the analyzer, thereby causing white state. However, when sufficient electric field is applied to the liquid crystal layer, since the incident light passes through the liquid crystal layer without changing its polarization, the light cannot pass through the analyzer, thereby causing black state.
As another example of a conventional LCD, U.S. Pat. No. 5,598,285 discloses an LCD, where two linear electrodes parallel to each other are formed on either of the two substrates, and a liquid crystal layer lies over the region between the two electrodes, and where the liquid crystal molecules are aligned parallel to the substrates. In this LCD, the potential difference between the two electrodes yields an electric field substantially parallel to the substrates and perpendicular to the two electrodes. The liquid crystal molecules are rearranged such that the torque due to the dielectric anisotropy and the elastical torque due to rubbing are balanced with each other. When the polarization directions of the polarizers are perpendicular to each other, in absence of electric field, the crossed polarizer blocks the incident light and makes the liquid crystal display to be in black state. However, when sufficient electric field is applied to the liquid crystal layer, the polarization of the incident light varies and the light passes through the analyzer, thereby causing white state.
The above-mentioned LCDs have disadvantages described hereinafter respectively.
The principal disadvantage of the TN-LCD is its narrow viewing angle. In the TN-LCD, the larger an angle made by the direction of the user""s eye and the direction normal to a surface of a display, the larger the value  n d where birefringence xcex94n is the difference of the refractive indices between in the direction of the long axes and the short axes of the liquid crystal molecules and d is the thickness of the liquid crystal layer. Accordingly, the contrast, which is defined as the luminance of the brightest state divided by that of the darkest state, abruptly decreases. In addition, gray inversion phenomenon also occurs. Accordingly, the viewing angle at which the contrast is equal to 10 is very narrow, and thus image quality is abruptly deteriorated when viewed at an angle larger then the viewing angle.
To compensate the viewing angle, methods using phase difference compensating films are suggested in U.S. Pat. No. 5,576,861, but they have disadvantages in manufacturing cost and the number of the process steps since the phase difference compensating films are additionally attached. Furthermore, the satisfactory viewing angle may not be still obtained even though the phase retardation compensation films are used.
The U.S. Pat. No. 5,598,285 has disadvantages in power consumption and aperture ratio. The LCD disclosed in the above U.S. Pat. No. 5,598,285 has an electric field of which strength is dependent on the positions, that is, the field strength is weaker as far from the electrodes. Therefore, in order to obtain sufficient field strength at the far point from the electrodes, high driving voltage is required. In addition, since all the electrodes are formed on one substrate and storage capacitors are formed to obtain sufficient capacitance, the aperture ratio is small.
In the meantime, since the liquid crystal display is a passive display, it requires an external light source. A white light is usually used for the light source of the liquid crystal display, and red, green and blue color filters are used for color display. The color filters are formed on one of the substrates, and a black matrix for preventing light leakage at the boundaries of the color filters is formed therebetween.
The light from the light source changes its properties, such as polarization, in the liquid crystal layer, and the transmittance of the light depends on the wavelength of the light. The transmittance also depends on the driving mode of the liquid crystal display.
In the case of TN LCDs, the transmittance of the blue light differs from those of the red and green lights by 10%. Moreover, the IPS LCD has the difference of the transmittances of the blue, red and green lights more than 40%.
In order to reduce the difference of the transmittance, two methods are conventionally used, one using a backlight unit and a driving circuit having additional characteristics and the other making a cell gap to be different for the pixels of different colors by adjusting the height of the color filters. However, the former method may increase the yield cost and the number of process steps, and the latter may cause uneven rubbing.
An object of the present invention is to obtain a wide viewing angle.
Another object of the present invention is to reduce power consumption of the liquid crystal display.
Still another object of the present invention is to enlarge the aperture ratio.
In order to accomplish the above-mentioned objects, the array of the electrodes of the LCD is modified.
First electrodes and second electrode insulated from each other are overlapped with each other at least in part. The second electrode forms a continuous plane between the first electrodes, and one pixel includes at least one first electrode and one second electrode.
The potential difference between the two electrodes generated by applying voltages to the electrodes yields an electric field. The shape of an electric line of force is semi-ellipse or parabola having a center on a boundary line or a boundary region between the first electrode and the second electrode, whereby the electric field on the electrodes has the vertical and the horizontal components.
The liquid crystal molecules on the first or the second electrode and on the boundary region between the two electrodes are re-arranged to have a twist angle and a tilt angle due to the vertical and the horizontal components of the electric field. Therefore, the polarization of the incident light varies by the rearrangement of liquid crystal molecules.
As described above, a wide viewing angle may be obtained since the liquid crystal molecules are re-arranged to have both the twist angle and the tilt angle.
In addition, the liquid crystal molecules on the first electrode and the second electrode contribute to displaying images since the electric field has the vertical and the horizontal components on the first electrode and the second electrode as well as on the boundary region between the two electrodes.
In addition, power consumption is low since the strength of the electric field is large on the boundary region between the first electrode and the second electrode.
In addition, aperture ratio may be enlarged since a storage capacitor for obtaining sufficient storage capacitance is not additionally required since the two electrodes are overlapped via an insulating film when using a thin film transistor (TFT) as a switching element.
Additional objects and advantages of the present invention are set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.